When clicking an annotation in the sidebar that corresponds to a
different chapter of a VitalSource book than the one that is currently loaded,
navigate the book to the chapter associated with the annotation.

 - Add `goToCfi` and `goToURL` to the set of `<mosaic-book>` APIs that
   the client uses, and implementations of these in the VitalSource EPUB demo.

 - Add optional `navigateToSegment` method for integrations, which
   triggers a navigation to the segment (eg. EPUB chapter) of a document
   that matches an annotation's selectors.

 - Add new `navigateToSegment` sidebar => guest RPC call, which forwards
   to the integration's `navigateToSegment` implementation.

 - Change `FrameSyncService.scrollToAnnotation` to accept an annotation
   rather than just a tag, so the service can conveniently match the
   annotation against the EPUB chapter information for the guest

 - Add logic in `FrameSyncService.scrollToAnnotation` to invoke the
   `navigateToSegment` RPC method instead of `scrollToAnnotation` if the
   annotation is in a different segment of the document than is
   currently loaded.

   Once the navigation to the new segment has completed, the document
   still needs to be scrolled to the annotation. FrameSyncService
   service handles this by recording the ID of the annotation in a
   `_pendingScrollToId` field, and then performing the scroll once the
   annotation anchors.